1. Field of the Invention
The present invention relates to a document processing apparatus such as a printer for externally taking in an image for processing.
2. Description of the Related Art
Conventionally, a printer, or a document processing apparatus, is connected to a communication medium such as a network. Document data is entered by an external host computer into the document processing apparatus. The entered document data is temporarily held in an auxiliary storage unit in the document processing apparatus. The temporary holding of the data is generally referred to as spooling. The spooling allows the document processing apparatus to adapt itself to the variation in the amount of data supplied from the host computer as well as the variation in the data transfer rate caused by the busy condition of communication line. Consequently, problems such as paper jamming on the printer can be handled only by the document processing apparatus.
The spooled data are sequentially read from the auxiliary storage unit, processed in the document processing apparatus to be converted to visible image data that is outputted onto medium such as paper. Generally, the auxiliary storage unit is large in storage size but slow in access speed, taking time in read/write operations. Therefore, it takes time to read spooled data from the auxiliary storage unit; especially, if the data processing time in the document processing apparatus is short, reading the data from the auxiliary storage unit forms a bottleneck in the flow of the entire processing of the document processing apparatus, sometimes retarding the processing.
To solve this problem, a technique was proposed as disclosed in Japanese Patent Non-examined Publication No. Hei-4-54525. In the disclosed method, a buffer of small storage capacity and an auxiliary storage unit having large storage capacity are arranged at the same time. Data is once written to the buffer. When the small-capacity buffer is emptied as the processing progresses, data is moved from the large-capacity auxiliary storage unit to the small-capacity buffer. Thus, processing delay is absorbed by the small-capacity buffer, realizing smooth data transmission.
However, when an attempt is made to implement the above-mentioned conventional technique on a color page printer in which data amount per page is very large, the small-capacity buffer cannot handle required data. Namely, in a page printer, an image to be outputted is generated for each page. Further, in a color printer, images are generated in the same number as the number of colors to be used, four colors of CMYK or RGB for example. Hence, a plurality of images are generated per page. Recently, image resolution has been increasing rapidly, requiring larger storage capacity per page. Consequently, a color page printer requires a storage capacity that is especially large. In the processing in which data amount is very large as compared with the number of pages to be outputted, it is required, beginning from the output processing of the first page, to perform data transfer from the large-capacity auxiliary storage unit to the small-capacity buffer a plurality of times. During the transfer processing, the output processing is interrupted, thereby diminishing the effect of small-capacity buffer installation. Increasing the storage size of the small-capacity buffer increases its production cost.
Spooling incessantly coming color information makes the small-capacity buffer always full to make the auxiliary storage unit always busy, thereby increasing the possibility of the conflict in timing between a write request by external retry and a request for reading from the buffer. If such a conflict occurs, the read operation is disabled, further lowering the processing speed.